Recent energy and environmental issues have drawn attention toward solar cells. From the standpoint of conserving resources, thin-film type solar cells will likely become mainstream. In general, devices in which a transparent conductive film, made of tin oxide (SnO2) for example, and an amorphous semiconductor such as amorphous silicon or amorphous silicon germanium serving as a photoelectric conversion layer are formed on a transparent substrate such as a glass sheet are used as thin-film solar cells.
Thermal decomposition methods often are used to form such transparent thin films, and transparent conductive films formed by such methods are polycrystalline. In the case of polycrystalline bodies of tin oxide, crystal growth proceeds as the transparent conductive film grows in thickness, resulting in the surface of the transparent conductive film becoming uneven. Thin-film solar cells are designed to exploit this unevenness in the surface of the transparent thin film and scatter incident light, and thereby increase the length of the optical path in the photoelectric conversion layer, exhibiting a so-called “light trapping effect.” Exhibiting this light trapping effect results in an improvement in the conversion efficiency of the photoelectric conversion device.
For example, JP S62-7716B discloses an amorphous silicon solar cell in which the average grain diameter in the surface of the transparent electrode is not less than 0.1 μm and not more than 2.5 μm, and mentions that the correlation between the size of the crystal grain diameter of the transparent electrode and the efficiency of the amorphous silicon solar cell was examined. However, the grain diameter generally exhibits the above-mentioned average grain diameter if a transparent electrode such as an indium oxide film or a tin oxide film is formed through a spray method or CVD (chemical vapor deposition), which are mentioned in this publication. The publication does not mention the ideal height of the crystal grain diameter or the distribution of the diameter or the height of the grains.
JP H2-503615A discloses a solar cell substrate that includes a conductive film having projections with diameters of 0.1 to 0.3 μm and a height/diameter ratio of 0.7 to 1.2 in its surface. However, this publication does not mention the distribution of the height of the projections or of the height/diameter ratio.
JP H4-133360A discloses a photovoltaic device that includes a tin oxide film whose surface has projections in the shape of truncated pyramids with heights of 100 to 300 nm and in which the angles between them and the normal of the principle surface of the substrate are 30° to 60°. However, this publication does not mention the distribution of the height of the projections or of the angles between the normal lines and the main surface of the substrate.
JP H3-28073B discloses a photovoltaic device having a translucent conductive oxide provided with an uneven surface with an average grain diameter of approximately 50 to 200 nm, a height difference of approximately 100 to 500 nm, and a spacing between projections of approximately 200 to 1000 nm. However, it does not mention the distribution of the grain diameters or of the height difference.
All of the photoelectric conversion devices disclosed in these laid-open publications are under the premise that amorphous silicon is used for the photoelectric conversion layer. Amorphous silicon as a material is well suited for the photoelectric conversion layer of photoelectric conversion devices because even at a low film thickness it exhibits high photoelectric conversion efficiency. The film thickness of photoelectric conversion layers made of intrinsic amorphous silicon is about 50 to 700 nm, whereas in the case of p-type and/or n-type conductive amorphous silicon layers in contact with the transparent conductive film, the film thickness is about 3 to 100 nm.
With a photoelectric conversion layer made of amorphous silicon, however, although the absorption coefficient of light at wavelengths shorter than its energy gap is large, the absorption coefficient of light on the long wavelength side thereof progressively diminishes as the wavelength increases. For this reason, most light on the long wavelength side is discharged outside the system of the photoelectric conversion device without being absorbed by the photoelectric conversion layer. Increasing the length of the optical path in the photoelectric conversion layer is the most effective way to increase the amount of light on the long wavelength side that is absorbed. Although the light trapping effect is exhibited somewhat even with conventional transparent conductive films, because there is such a strong relationship between the shape of the unevenness in the surface and the wavelength of the light, a shape that effectively scatters light in the absorption region of amorphous silicon is not necessarily also effective for light at long wavelengths. In other words, there is a separate shape that is suited for dispersing light at long wavelengths for the unevenness in the surface of a transparent conductive film.
In recent years, photoelectric conversion layers constituted by amorphous silicon-germanium, thin film polycrystalline silicon, or microcrystalline silicon, for example, in place of amorphous silicon have started to come into use. These absorb much of the light at wavelengths longer than the visible spectrum region, which is the absorption region of amorphous silicon.
Consequently, future increases the conversion efficiency of solar cells will require transparent conductive films that can effectively scatter light of wavelengths longer than the absorption region of amorphous silicon.
Conventionally, due to inadequate research on the relationship between the surface shape of transparent conductive films and the wavelength of the scattered light, it was thought that it was important to scatter light of wavelengths below the absorption region of amorphous silicon. This ultimately resulted in a tendency toward greater uniformity in the surface unevenness of transparent conductive films. When the surface unevenness of a transparent conductive film is made uniform, the reflectance of the transparent conductive film is increased, and if the transparent conductive film has a high reflectance, then light cannot sufficiently enter the photoelectric conversion layer because incident light that has been transmitted through the transparent substrate is reflected by the transparent conductive film. This resulted in the problem of a drop in the conversion efficiency of the photoelectric conversion device.
The present invention has an aspect that was arrived at in light of the above problems. It is an object of this aspect, at least in a preferable embodiment of the invention, to provide a transparent conductive film with which light of wavelengths longer than the absorption region of amorphous silicon can be scattered effectively and which itself has a low reflectance. It is a further object to provide a photoelectric conversion device that is furnished with this transparent conductive film and that has improved photoelectric conversion efficiency. Hereinafter, the invention having this aspect is referred to as the “invention of the first embodiment.”
The above-mentioned publications frequently mention the shape of the surface of the transparent conductive film, however, they do not touch upon a case in which the projections are of different shapes or mention their size. A transparent conductive film having many projections with trapezoid-shaped cross sections has increased reflectance, lowering the amount of incident light on the photoelectric conversion layer. When the trapezoid-shaped projections are large, parts of the p-type amorphous silicon layer of the photoelectric conversion layer in contact with the transparent conductive film become thin, leading to instances in which shorts occur between the transparent conductive film and the i-type layer formed on the p-type layer. Also, the thickness distribution of the p-type layer becomes nonuniform, which at times also leads to nonuniformity at the p-i junction or the in junction. As a result, there is a drop in the characteristics of the photoelectric conversion device.
The present invention has another aspect that was arrived at in light of these problems. It is an object of this aspect, in at least a preferable embodiment of the invention, to provide a high performance photoelectric conversion device, and a transparent conductive film used in that photoelectric conversion device, in which the reflectance of the transparent conductive film is kept low by reducing the proportion of projections regarded as having trapezoid-shaped cross sections, and in which shorts are prevented by reducing the number of large trapezoid-shaped projections. Hereinafter, this is referred to as the “invention of the second embodiment.”
When there are defects such as pinholes in even one of the p-type, i-type, or n-type layers in the photoelectric conversion layer, shorts may occur and the photoelectric conversion efficiency drops noticeably. When the p-type and/or n-type layers are increased in thickness in order to prevent such defects from occurring, the amount of light that is absorbed in those layers is increased, reducing the amount of light that is incident on the i-type layer, which is the photoelectric conversion layer. Also, it leads to nonuniformity at the p-i junction or the i-n junction, which lowers the photoelectric conversion efficiency. Consequently, in light of this problem, the transparent conductive film should be given a level surface. On the other hand, if the light trapping effects are to be exploited, then it is conceivably preferable that the surface has numerous large protrusions in order to raise the photoelectric conversion efficiency and in particular to increase the effect of scattering long wavelength light.
After intense research regarding the shape of a transparent conductive film surface that adequately achieves these conflicting desirable characteristics, the present inventors found that a state in which there are both few very large projections that protrude out and also numerous relatively large projections is ideal.
In other words, in yet another aspect of the invention, in at least a preferable embodiment thereof, it is an object to provide a transparent conductive film having tin oxide at its main component and an ideal surface shape to serve as the thin film electrode of a photoelectric transducer element. Hereinafter, this invention is referred to as the “invention of the third embodiment.”